add AI contribution policy to CONTRIBUTING.md (#561)

Co-authored-by: Florence Bockting <florence.bockting@aalto.fi>

Author: florence-bockting

.github/CONTRIBUTING.md

@@ -54,3 +54,5 @@ new to you.
 Please note that the projpred project follows the Stan project's 
 [Code of Conduct](https://discourse.mc-stan.org/t/announcing-our-new-stan-code-of-conduct/23764).
 By contributing to this project you agree to abide by its terms.
+
+All contributions must follow the [Stan AI Contribution Policy](https://github.com/stan-dev/stan/wiki/AI-Contribution-Policy).